The present invention relates to a fail over method used in a computer system where servers are booted from an external disk.
In general, a server boots an OS (Operating System) installed in a built-in disk device. A system using this configuration conventionally has a redundant configuration in which the OS and the applications on the disk built in a server are installed on the disks of a plurality of servers for use in taking over a task when the server fails. In this method, when an operating server (primary server) executing a task fails, another server (standby server), in which the same OS and the applications are installed, is started to take over the task. However, this method requires each primary server to have a standby server with which to pair, increasing the cost and manpower required for the initial installation.
Another method is that a server uses an external disk array device as a disk device for booting. In this case, because the disk array device can be connected to a plurality of servers via a fiber channel or a fiber channel switch, the boot disk of a server connected to the disk array device can be referred by another server. Therefore, even if a primary server executing a task fails in this configuration, a standby server can be started using the boot disk of the primary server to take over the task. Because the content of the boot disk is taken over directly to the standby server in this method, the OS and the applications are taken over directly. In addition, this method eliminates the need for preparing a standby server with which to pair, allowing the task to be taken over from any primary server to any standby server. The problem with this method is that, when the hardware configuration of the primary server differs from that of the standby server, the OS or the applications on the boot disk sometimes do not operate properly.
The conventional method solves this problem by requiring the user to set up a standby server with the hardware configuration equivalent to that of the primary server assuming that both systems should have the equivalent hardware configuration. However, this method increases user's manpower for the initial installation. In addition, because the task can be taken over in some cases even if there is a slight difference in the hardware configuration such as the CPU speed or the memory capacity, a server with the same hardware configuration need not always be prepared.
In addition, when the partition function is used where a plurality of servers are configured virtually as one logical server as shown in U.S. Pat. No. 6,510,496, the partition configuration of a primary logical server must be matched with that of a standby logical server. However, once a standby logical server is set up in such a way that its partition configuration matches with that of a primary logical server, the standby logical server cannot take over a task from some other primary logical server that has a different partition configuration. This requires the user to prepare a standby logical server that always forms a pair with the primary logical server, increasing the initial installation cost and the initial buildup cost.